In many ways, those who have an SCT profile have the opposite symptoms of those with classic ADHD: Instead of being hyperactive, extroverted, obtrusive, and risk takers, those with SCT are passive, daydreamy, shy, and "HYPO"-active in both a mental and physical way. They also don't have the same risk factors and outcomes. Their demeanor is sluggish, as if "in a fog" and logically they also process information more slowly. A key behavioural characteristic of those with SCT symptoms is that they are more likely to appear to be lacking motivation. They lack energy to deal with mundane tasks and will consequently seek things that are mentally stimulating because of their underaroused state. Those with SCT symptoms show a qualitatively different kind of attention deficit that is more typical of a true information input-output problem, such as memory retrieval and active working memory. Conversely, those with the other two subtypes of ADHD are characteristically excessively energetic and have no difficulty processing information.

Diagnosis

Since the symptoms of SCT are not recognized in any standard medical manuals, those who have significant SCT symptoms would likely receive an ADHD/PI diagnosis. Currently the APA is considering the inclusion of SCT in the DSM-V, which is scheduled to be released in 2012.

Causes

Like ADHD, those with SCT symptoms have a condition that appears to be genetic in nature. Far less is known about this group yet the impairments seem to indicate the prefrontal cortex region of the brain and difficulties with working memory. The 7-repeat allele polymorphism of the DRD4 gene is also linked more strongly to this group than to ADHD/C and ADHD/PHI subgroups.

It is thought that SCT, ADHD-PI, and ADHD are due to variations in the availability of dopamine and norepinephrine, and/or the efficiency of the large chemical structures of the specific receptors and re-uptake receptors. This would explain the efficacy of stimulants such as amphetamines on the treatment of ADHD and SCT.

Treatment

Up to 90% of children with ADHD respond well to methylphenidate (Ritalin) at medium-to-high doses, however, a sizable percentage of children with ADHD-PI do not gain much benefit from Ritalin, and when they do benefit, it is at a much lower dose. Tests in lab rats have demonstrated that low doses of Ritalin can increase norepinephrine levels.

Those with ADHD-PI often respond well to amphetamines, such as the prescription medication Adderall. While methylphenidate and amphetamines have many similar effects on patients (both inhibit reuptake of the neurotransmitters dopamine and norepinephrine, for example), amphetamines also promote release of those neurotransmitters. This positive effect appears to support the hypothesis that SCT is related to neurotransmitter deficiencies.

Prognosis

ADHD is a developmental disorder, meaning that certain traits will be delayed in the ADHD individual. These traits can and usually will develop in people with ADHD, but just at a much slower rate than the average person. With ADHD, it has been estimated that this lag could be as high as thirty to forty percent in the development of certain skill sets, such as selective attention. Symptoms of ADHD are often seen by the time a child enters preschool. Those with SCT symptoms typically show a later onset of symptoms in comparison to ADHD. They have greater difficulty with academic tasks and far fewer social difficulties when compared to those with combo and hyperactive ADHD.

Selective attention difficulties of those with SCT manifests itself academically, in that they are prone to making more mistakes while working. Those with classic ADHD do not have this difficulty. Those with SCT have difficulty with verbal retrieval from long term memory, but may have greater visual spatial capabilities. They have deficits in working memory which has been described as the ability to keep multiple things in mind for manipulation, while simultaneously keeping this information free from internal distraction. Consequently, mental skills such as calculation, reading, and abstract reasoning are often more challenging for those with SCT. They also have a more disorganized thought process, a greater degree of sloppiness, and lose things more easily. They tend to have a greater degree of comorbid learning disabilities. Instead of having greater difficulty selecting and filtering sensory input, as is in the case of SCT, people with other types of ADHD have problems with inhibition.

Studies indicate that comorbid psychiatric problems are more often of the internalizing variety with SCT, such as anxiety, depression, and social withdrawal. Their typical shy nature and slow response time has often been misinterpreted as aloofness or disinterest by others. In social group interactions, those with SCT may be ignored. Those with the other types of ADHD are more likely to be rejected in social situations, because of more intrusive or aggressive behavior. Those with classic ADHD also show externalizing problems such as substance abuse, oppositional-defiant disorder, and, to a lesser degree, conduct disorder.

Prevention

There is no known way to prevent ADHD/PI. Some studies indicate an association between mothers who smoke during pregnancy and a higher rate of ADHD in their children. Avoiding smoking, alcohol, and drugs during pregnancy may help reduce the risk of developing ADHD or similar behaviour in offspring.

History of the term SCT and its relationship to the DSM

Sluggishness, drowsiness, and daydreaming were the characteristics listed in the DSM-III (in use from 1980-1987) that were to also be present in the diagnosis of Attention Deficit Disorder (ADD) without Hyperactivity. In a study looking at these symptoms (Lahey et al., 1988) the authors stated, "these symptoms were statistically extracted as a distinct factor", coined, Sluggish Cognitive Tempo. The Sluggish Tempo factor was found to correlate significantly to the Inattention factor, but only when Hyperactivity-Impulsivity symptoms were absent.

Sluggish Cognitive Tempo symptoms were removed from the Inattention symptom list in 1988 because of poor negative predictive power for the inattentive subgroup, and because DSM contributors and editors wanted the inattentive symptoms to be identical for all ADHD subgroups. The presence of the SCT symptoms tended to predict inattention, but the absence of these symptoms did not predict the absence of inattention. This analysis did not take into account the possibility that the SCT symptoms could help predict a distinct grouping within the ADHD/PI subgroup and that the ADHD/PI subgrouping could be heterogeneous in nature.

In the DSM-IV, with its new classification of symptoms for predominately inattentive ADHD, 50 to 70% of those with a ADHD-PI diagnosis have subclinical levels of hyperactivity-impulsiveness symptoms. People with ADHD combined type (ADHD-C) and predominantly hyperactive–impulsive type (ADHD-PHI) may outgrow some, or most of their hyperactive symptoms during or after childhood, while inattentive symptoms typically remain into adulthood. In contrast, those with SCT have had only inattentive features from a young age with little to no history of hyperactivity-impulsiveness. Dr. Russell Barkley has proposed that the DSM-IV designation of ADHD-PI be used only for those displaying purely inattentive symptoms and that those who have had a history of any hyperactivity be designated as ADHD combined subtype. Currently, one can have a few hyperactive symptoms and still receive a diagnosis ADHD-PI. Others believe that SCT should be classified as a new separate disorder when the DSM is next updated.

Relationship to dysexecutive syndrome

The Executive system of the human brain coordinates actions and strategies for everyday tasks. Dysexecutive syndrome is defined as "cluster of impairments generally associated with damage to the frontal lobes of the brain" which includes "difficulties with high-level tasks such as planning, organising, initiating, monitoring and adapting behaviour.

Adele Diamond has recently postulated that the core cognitive deficit of those with ADHD-PI (ADD), is working memory, or, as she coined in her recent paper on the subject, "childhood-onset dysexecutive syndrome". She states:

"Instructional methods that place heavy demands on working memory will disproportionately disadvantage individuals with ADD".

"language problems often co-occur with ADD, and it is suggested that part of the reason might be that linguistic tasks, especially verbal ones, tax working memory so heavily. Spatial and artistic skills, however, are often preserved or superior in individuals with ADD."

"The working memory deficit in many children with ADD is accompanied by markedly slowed reaction times, a characteristic that covaries with poorer working memory in general".

"Individuals with ADD have difficulty maintaining a sufficiently high level of motivation to complete a task...They go looking for something else to do or think about because they are bored...to remedy a general lower arousal level.."